A commonly used approach to control internal pressure within storage tanks is to employ a valve having a valve seal plate that is slidably mounted on a stationary guide post. This approach relies on sliding engagement between the seal plate and the guide post to guide the seal plate during opening and closing of the valve. Several problems are encountered with such an arrangement. For example, it is difficult to correctly establish the internal tank pressure at which the valve will open (e.g., the valve opening set point). Additionally, it is difficult to assure adequate discharge volume without attendant and excessive internal tank pressure build-up. Moreover, the seal plate may stick or become bound on the guide post during opening and closing of the valve.
Spring-operated valves are sometimes used to limit internal tank pressure. This approach relies on a compression force exerted by a spring onto the mid-portion of the seal plate to keep the valve sealed during normal pressurization of the tank. As the valve opens, the spring is compressed and exerts a closing force onto the seal plate which is higher than the initial valve opening set point. The more the valve opens, the greater the closing force exerted by the spring onto the seal plate. As a result, these types of spring-operated valves will never fully open to allow maximum flow through the valve unless internal tank pressures are exceedingly high. Additionally, these types of spring-operated valves will close whenever the internal tank pressure is reduced to a point approximately equal to the initial valve opening set point. Accordingly, spring-operated valves will repeatedly open to less than maximum flow and will tend to prematurely reseal or close, thereby creating waves of back pressure and resistance throughout the fluid transfer system.
Weight-operated valves have also been used to limit internal tank pressure. These types of valves rely on a relatively large weight positioned at the mid-portion of the seal plate to keep the valve sealed during normal pressurization of the tank. Since the location of the weight remains unchanged, the closing force exerted onto the seal plate will remain constant and the closing force exerted onto the seal plate will always be approximately equal to the initial valve opening set point. Similar to spring-operated valves, weight-operated valves will never fully open to allow maximum flow through the valve unless internal tank pressures are exceedingly high. Likewise, weight-operated valves will close whenever the internal tank pressure is reduced to a point approximately equal to the initial valve opening set point. Accordingly, weight-operated valves will repeatedly open to less than maximum flow and will tend to prematurely reseal or close, thereby creating waves of back pressure and resistance throughout the fluid transfer system.
Thus, there is a general need in the industry to provide an improved safety relief valve for use in association with a liquid storage tank. The present invention meets this need and provides other benefits and advantages in a novel and unobvious manner.